One of the greatest advances in concrete technology in recent decades has been advent of air entrainment. Today the use of entrained air is recommended in concrete for nearly all applications.
Probably the most oft-cited reason for using intentionally entrained air is to improve the resistance of concrete to alternate freezing and thawing. Indeed, tests have proven that air entrained concrete containing minimum recommended air contents on the order of about 5 to 7.5%, .+-.1%, air content (expressed as a percent by volume) will withstand up to about 1900 freeze-thaw cycles as contrasted to a maximum of about 150 cycles of a non-air entrained concrete which is identical in all other respects. See for example, "Air-Entrained Concrete", Portland Cement Association, Document ISO45.02T, 1967.
There are many other benefits from the use of air entrained concrete including improved workability, increased resistence to de-icers such as calcium chloride, increased sulfate resistance, and improved water tightness.
One widely used method of making air entrained concrete includes the step of adding an air-entraining admixture during the mixing of the concrete. Experience has indicated that the mixing action is the most important factor in the production of air-entrained concrete, and in this regard uniform distribution of entrained air voids is essential to the production of scale-resistent concrete; non-uniformity is always a risk if the entrained air is inadequately dispersed during mixing. Such factors as the batch size of the concrete being mixed, the condition of the mixer and the rate of mixing are also important. Over mixing may even result in a loss of some of the entrained air, but the techniques and preferred procedures associated with the mixing phase of air entrained concrete are now rather widely understood, and further amplification is considered unnecessary for those skilled in the art.
A number of air-entraining admixtures manufactured from a variety of materials are commercially available today. One readily available air-entraining admixture is AMEX, a product available from American Admixtures Corporation, which is obtained in the production of a neutralized vinsol resin solution.
Indeed, vinsol resins are undoubtedly the most widely used air-entraining admixture in the United States, although other air-entraining products are also available.
The air-entrained concrete which results from the use of recognized air-entraining products contain a large number of air bubbles of an extremely small size; average bubble diameter usually ranges from three thousandths to six thousandths of an inch and as many as three hundred to five hundred billion bubbles may be present in a cubic yard of air-entrained concrete having an air content in the range of four to six percent by volume, and one and one-half inch maximum sized aggregate. The bubbles are not interconnected and are well distributed throughout the cement/water phase. The spacing of the air voids is an important factor in the freeze-thaw durability of hardened concrete, and a spacing of less than 0.008 inches, as measured by ASTM C457 standard, is considered essential for the attainment of the requisite freeze-thaw resistence.
One of the most significant developments in concrete since air-entrained concrete was developed in the mid-1930's is the use of so-called super plasticizers.
Super plasticizers are chemical compounds which, when added to concrete, fluidize the concrete for a period of time so that (1) normal workability can be obtained in concrete having much lower water-cement ratios then would normally be employed or (2) extremely workable "flowing concrete" (that is essentially self-leveling without undesirable side effects, such as segregation, low-durability, low abrasion resistence, and bleeding,) can be obtained, or (3) a combination of (1) and (2). One commercially available product is MELMENT which is obtainable from American Admixtures Corporation, and which is a modified polycondensate product of melamine and formaldehyde, and does not contain any calcium chloride or any other accelerating salts.
Super plasticized concrete is extensively used in cast-in-place concrete work where extreme flowing characteristics are required such as in areas of high density of reinforcement, pumping, and in complicated form work.
Among the advantages of the use of super plasticizers in pre-cast and readi-mix concrete are (a) increased strength at all ages, (b) improved resistance to attack by sulphates, (c) increased bonding to reinforcing steel, (d) improved workability and formability, and (e) reduced permeability to water penetration.
When MELMENT is added to a concrete mix, the plasticizing effects last for approximately 30-60 minutes, depending on the job conditions. Consequently it should be added at the job site when used in ready-mixed concrete.
MELMENT is normally batched at 35-70 fluid ounces per cwt of Portland Cement for pre-cast work and at 20-40 fluid ounces per cwt of Portland Cement for ready-mix concrete. An additional dose may be added to a batch to restore flow characteristics which may have dissipated due to failure to place the batch within the normal fluidizing period.
The literature in the trade states that several chemicals purport to have the high workability characteristics required in a super plasticizer, but two main groups are well recognized. They are (1) sulfonated melamine formaldehyde condensates, and (2) sulfonated napthalane formaldehyde condensates. Additionally, certain modified lignosulfonates are said to super plasticize. At least twelve proprietary super plasticizers are currently available in the world, eight of which belong to the two categories (1) and (2) immediately above.
Super plasticized concrete may be placed rapidly and easily and, frequently, without vibration or, at least with reduced vibration over what would normally be required.
Additional characteristics of super plasticized concrete are set out in "Super Plasticized Concrete," ACI Journal, May, 1977, , pp. N6-N11 inclusive, and the references set out therein.
Although concrete which may be classified as either air entrained or super plasticized has proved eminently feasible for many applications requiring only the qualities attributable to air entrainment or super plasticization, difficulties have been encountered when the contractor has attempted to use both an air-entraining admixture and a super plasticizer.
Specifically, it is today an universally accepted fact that the air-void system of hardened air-entrained concrete containing super-plasticizers and neutralized vinsol resin is very poor; that is, the air-void spacing factor is greater than 0.008 inches. As mentioned, the air-void parameters, and specifically the spacing factor of the air-void system, is a major criteria for predicting the probable performance of concrete to withstand repeated freeze-thaw cycling.
The problem, then, faced by the industry is to produce a concrete which possesses the desirable freeze-thaw and allied characteristics of air entrained concrete, together with the excellent workability and increased strengths of super plasticized concrete.
Accordingly a primary object of the invention is to provide an air-entrained, super plasticizing additive for concrete.
Another object is to provide a concrete having the desirable freeze-thaw characteristics, and allied characteristics of air-entrained concrete, together with the excellent workability, increased strengths, and allied characteristics of super-plasticized concrete, and a method for producing same.